1. Field of the Invention
The present invention relates generally to the art of optical inspection of specimens, such as semiconductor wafers, and more specifically to a massively parallel inspection system having relatively large fields of view.
2. Description of the Related Art
Optical inspection techniques for small specimens, such as features on semiconductor wafers, have previously utilized confocal microscopy to locate and examine the desired site. Confocal microscopy or confocal imaging each provide a variety of limitations, most particularly a relatively narrow field of view, or spot width, depending on the desired resolution of the image received and the overall quality of the components employed.
The essence of confocal microscopy or confocal imaging is to perform a double imaging of the specimen utilizing point illumination and a point detector. For example, the use of confocal imaging entails focusing a beam transmitted and reflected on the semiconductor wafer using an objective lens, wherein the area examined at any given time is on the order of 20-50 microns on each side. The problem with this narrow field of view is that inspection of a wafer having dimensions in excess of 100 square inches can be extremely time consuming.
Further, this single spot inspection system, wherein inspection comprises scanning using a single narrow spot, requires two dimensional scanning. In such an arrangement, the single spot passes over a linear portion of the sample while recording data, then moves one spot diameter or a fraction of a spot diameter over and passes over the next linear portion of the sample while recording data. The system thus iteratively progresses through inspection of the wafer using this stepping technique.
Different methodologies have been employed to improve the throughput of confocal microscopy and inspection systems, but these systems typically address improving scanning speed and accuracy using the smaller spot width mentioned above. The use of large spots removes much of the confocal advantage and is therefore undesirable.
Systems employing beam expanders have been utilized previously, but such systems have been directed toward alignment issues. Such a system is illustrated in U.S. Pat. No. 5,231,467 to Takeuchi et al. The Takeuchi system provides a compensating optical system which utilizes a diffraction grating on a semiconductor wafer to diffract the incoming light beam. However, this does not significantly alter the area on the specimen which can be observed in a given time period, but rather provides position alignment assistance when different wavelength light is used on the two samples to be aligned.
Further, systems employing multiple spot confocal imaging have also been available. U.S. Pat. No. 5,737,084 to Ishihara illustrates a system in which a multi-spot confocal arrangement is used to measure three-dimensional shapes. U.S. Pat. No. 5,248,876 to Kerstens et al. describes a similar system for imaging at different heights. In both these systems an array of pinholes is utilized both with and without an accompanying lens array in front of an extended source of light to generate a plurality of point sources. These point sources are then brought into focus on the sample by the action of an objective lens. The reflected beams are directed back through the same pinhole array, or another matching array of pinholes. One drawback of such an approach is that the illumination position and collection pinholes must be precisely matched over the entire array. This requires a very high degree of mechanical stability, and implies a relatively large sensitivity to unwanted vibrations. Even in a situation where the illumination and collection pinhole arrays are one and the same, as in the case of U.S. Pat. No. 5,737,084, the relative size of the pinholes with respect to the eventual pixel size in the detector array still requires precise alignment between the detector and pinhole arrays. A further problem with the identical illumination/pinhole array arrangement is the possibility of stray light that may find its way onto the unintended detector pixels. Special precautions are thus necessary to eliminate this risk. In addition, the patent only describes a system capable of generating a sampled version of the image of the object under examination. No provisions are made for the scanning of either the beams or the sample to cover the entire sample, and no methodology is taught to achieve the same. It is important to note that the scanning action in a multi-spot system should ideally be different from that practiced in the case of a single spot system so that proper advantage is taken of the multiplicity of the beams.
Yet another consideration that pertains to these references is the fact that they rely on a precise action of the microlens/pinhole structures to generate high quality beams for each individual focal spot. This precision may not be easily attainable, particularly for applications in very high resolution imaging, where the quality of the wavefront in each beam is paramount.
An important aspect of current scanning applications is the operation of such a system as a dark-field imaging/inspection system. This issue is addressed in U.S. Pat. No. 5,248,876, which teaches the use of polarization, and polarization rotating components, to separate scattered radiation from the spectrally reflected light. Such an approach has undesirable consequences in that it essentially transforms the system into a polarizing microscope and requires the use of polarized light of a given direction to image the specimen. This polarization method also requires the use of two orthogonal polarizers in an arrangement, with one being positioned annularly with respect to the other, which forms the central region of a circular aperture. In dark-field imaging/inspection, the ability to respond to all features regardless of the specificity of their orientation or polarization behavior is extremely important. The inability to provide this feature is a significant drawback.
The references cited above also only address usage of multiple normally, or nearly normally, incident beams with respect to the sample. They do not teach the methodology under which the multi-spot arrangement could be used in obliquely incident configurations, where great advantage is gained in detecting minute defects on the surface of specimens such as silicon wafers.
It is therefore an object of the current invention to provide a system for increasing the inspected area of a confocal microscopic system, thereby increasing throughput, while at the same time offering minimal optical or confocal degradation under both brightfield and darkfield scanning.
It is a further object of the current invention to provide a system for optical inspection which has improved throughput performance and does not require significant modifications to existing hardware or software.
It is still a further object of the current invention to provide a system for detecting defects on a semiconductor wafer, the system having the ability to detect defects in a relatively short time period.
It is yet another object of the current invention to provide an optical inspection system which is relatively impervious to mechanical vibrations and stray light, and does not require precise alignment between detector and collection array.
It is yet another object of the current invention to provide a darkfield imaging optical inspection system which does not require use of polarized light of a given direction in the imaging process, and responds to all features irrespective of the specificity of their orientation or polarization behavior.
According to the present invention, there is provided a massively parallel inspection and imaging system which employs a plurality of focused beams to illuminate the sample. Light energy is emitted from a laser and passes through a relatively low resolution diffraction grating or preferably a digital optical element (multi spot generator). The low resolution diffraction grating concentrates the transmitted energy into multiple discrete directions or orders. The low resolution diffraction grating can be either a linear or two dimensional grating. The beams split by the diffraction grating pass through a telescope and are recombined onto a beamsplitter/scanner and are diverted toward the specimen. On reflection downward, the beams diverge again toward a focusing objective, wherein the focal plane of the focusing objective coincides with the apparent plane of light splitting.
The resultant light thus consists of a number of focused beams, and the sample is illuminated by these beams simultaneously. Upon reflection of the light from the sample, the light passes back through the focusing objective in a number of beams, and the beams converge toward the beamsplitter. The beamsplitter reflects a portion of the energy in the collimated beams. The collimated beams pass through a focusing lens, which brings all beams onto foci on a detector array, which may be one or two dimensional discrete photodiodes, CCD, or any other reasonable detection array.
Scanning in the system may occur in various ways. The system may pass the sample iteratively by scanning it beneath the objective in a raster fashion in the x and y directions, or alternately by spinning the sample about a central axis. Using these types of scanning methods, the position of the focused spots on the CCD or sensor array elements is fixed. Alternately, the system may employ a mechanical raster scanning arrangement of the beams using a scanning arrangement such as a resonant mirror scanner. In this case, both the sample and the beams are mechanically rastered. In this situation, for example, the beams may repeatedly move backward and forward in the x direction while staying fixed in the y direction, while the sample makes smaller movement in the y direction and does not move in the x direction.
Another potential scanning technique for the current invention may be used during the inspection of a patterned wafer. Using motion of both the scanning beam and the wafer, movement of each must be coordinated to properly observe the Manhattan xe2x80x9cstreetsxe2x80x9d, or tracks of wiring, located on the wafer. To achieve proper orientation and observation, the stage speed in the cross direction is set at the ratio of the distance between the first and last lines divided by the period of the scanner. When the wafer reaches the end of a swath, the scanning must occur in the opposite direction. The same strategy as described above may be employed, with the exception of a reversal of the fast scanning direction, or a re-orientation of the stage or scanner to provide the reverse angular disparity between the scanner and the wafer. This reverse scanning technique may be employed by using a bidirectional scanner such as a galvo mirror and taking data on the reverse path across the wafer. Alternately, the stage may perform a fast return to one swath width plus one pixel from the position where the prior scan began, and the next swath of data is then taken in the same manner as before.
Other objects, features, and advantages of the present invention will become more apparent from a consideration of the following detailed description and from the accompanying drawings.